The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Govt to keep asylum seeker docs secret

Australian government lawyers are fighting to keep secret documents used in the decision to detain 153 asylum seekers at sea.

Commonwealth barrister Stephen Donaghue QC told the High Court that some documents would be subject to public interest immunity, meaning they would not have to be revealed in court.

The asylum seekers have been detained on the high seas, outside Australia's migration zone, since their boat was intercepted on its way from India on July 7.

Dr Donaghue could not say whether the government would admit or deny a decision had been made to take the asylum seekers to a place other than Australia.

Human Rights Law Centre executive director Hugh de Kretser said there was "extraordinary secrecy" around the government's decisions.

"(The asylum seekers) simply ask, through this case, that those decisions be made fairly," Mr de Kretser told reporters in Melbourne on Friday.

Dr Donaghue said the government had no plans to send the asylum seekers to Sri Lanka, and would stick to its undertaking that no one on the boat would be sent to another country without three days written notice.